Nitrogen removal from natural gas



Dec. 8, 1964 J. BROCOFF ETAL NITROGEN REMOVAL FROM NATURAL. GAS

Filed Feb. 6, 1961 4 2 Par a 529-90? IN REFERENCE TO THE ABOVE SYMBOLS; "Iv" INDICATES A THROTTLE VALVE.

"PC.\/."|ND|cATEs A PRESSURE CONTROL VALVE.

L.C.V"|No|cATl-:s A LEVEL CONTROL VALVE.

rl cz B20 co FF RHLPH 5. SW/NGHOLM IN V EN TORS United States Patent 3,16%,4fi9 NITROGEN REMOVAL FRQM NATURAL GAS Jack Brocoff, Fullerton, and Ralph S. Swinghoim, Anaheim, Caliih, assignors to The Finer @orporation, Ltd, Los Angeles, Qalih, a corporation of alifornia Filed Feb. 6, 1961, Ser. No. 87,242 16 Claims. (Cl. 62 m) This invention relates to the removal from light hydrocarbon gases undergoing liquefaction of so-called inert gases such as hydrogen, helium, argon oxygen, nitrogen, and any other gases having boiling temperatures lower than methane. Since most generally, nitrogen removal presents the major problem, the invention will be described with reference to the separation of nitrogen typically from natural gases where the primary object is to obtain liquid methane. The invention is not limited to natural gas but can apply to any light hydrocarbon gas containing inerts.

In any process for liquefying natural gas, the methane and heavier hydrocarbons can be condensed by cooling the compressed gas below its boiling point. The presence of gases having boiling temperatures below that of methane, necessitates cooling the gas to lower temperature than otherwise would be necessary for methane condensation. This requires'additional expenditure of energy per unit of liquefied product obtained.- Moreover, if the inerts were not removed, their concentration would increase in the refrigeration cycle to the point where liquefaction of the product would cease.

Heretofore it has been proposed to effect an initial methane condensation by cooling the compressed natural gas, and to vent or otherwise remove from the system the uncondensed inerts together with considerable quantities of methane upon which work has been done and energy expended in the compression and cooling. Alternatively the purge gas has been further processed in a distillation column. Such practices have been wasteful in terms of potential methane recovery in the case of the former and energy utilization in the case of the latter.

Our general object is to provide for greater efiiciencies and economies in methane recovery from natural gas, by utilizing the energy potentials in a system involving nitrogen separation from refrigerated compressed natural gas at a rate equal to the nitrogen input rate, to controllably govern the methane content of the purged nitrogen stream in accordance with such methane content thereof as may be desired or needed for specific purposes, e.g. for fuel gas, while retaining in the system to final recovery, methane in excess of that desired quantity and which ordinarily would be lost from the system.

More specifically our objective is to utilize a combination of eifects and conditions, including the temperature and pressure of the initially separated nitrogen stream, cooling by expansion thereof, and heat interchange, as required, with low temperature methane in the system, to condense and return to the system for final recovery, methane in the separated nitrogen which would be wasteful in the purged gas.

The invention, and the manner in which these and other objects are accomplished will be understood from the following description of an illustrative process embodying the invention, shown in flow sheet form by the accompanying drawing.

Referring to the drawing, the feed gas is delivered to the system through line 10 and compressor 11, where necessary, at a pressure typically in the range of 400 to 1500 p.s.i.g. The feed gas then is cooled by appro-j priate refrigeration, as by conventional cascade cooling, at the refrigeration stage diagrammatically indicated at 12 and involving indirect heat interchange between the feed gas and a suitable refrigerant indicated to be passed through the refrigeration stage by way of line 13. Being precooled to a temperature that may be in the range of about 80 to l P. so that the feed gas is converted essentially to liquid phase, the refrigeration stage efliuent is discharged through line 14 and pressure re duction valve 15 into a separating chamber 16 from which predominently methane condensate is withdrawn through line 17.-

For the purpose of ultimately producing the methane condensate at a temperature sufliciently low to permit storage at practicably low pressures, we may employ a sequential flash and cascade system of adiabatic'cooling, involving typically three flash stages beyond the separating zone 16. Thus the condensate in line 17 is flashed by pressure reduction at valve 18 into chamber 19, the vapor overhead from which passes through line 20 to be compressed and returned by compressor 21 through line 22, compressor 23 and line 24 into the feed gas stream entering the refrigeration stage 12 through line 10. Similarly, the condensate from chamber 19 is taken through line 25 to be flashed past valve 26 into chamber 27, the overhead from which passes through line 271 to compressor 28 for discharge through line 29 into line 20. Beyond chamber 27, the condensate is discharged through line 30 and flashed past valve 61 into storage tank 32, the vapor overhead from which is taken through line 33 to compressor 34 and returned to line 271.

in chamber 19, 50 p.s.i.a. and minus 233" F. in chamber 37, and 15 p.s.i.a. and minus 255 F. in the'storage tank 32. p a a The invention is more particularly concerned with the treatment given the gaseous effluent from the separating zone 16 removed through line 36. Heretofore, the usual practice has been to purge all or part of such efiluent from the system, and in so doing to accept an uneconomical loss because the purge necessarily involvesa loss of product on which work hasbeen done, and a loss of energy which otherwise could be available for further cooling and condensation of the'feedgas. The present invention departs from such uneconomica-l practices in that we take to ultimately purge only that por-- tion of the vapor in line 36 which contains the quantity of inerts entering with the feed gas, and limit the amount of combustible gas in the mixture going to purge, to an economical ratio which usually isthat which can eco nomically be used in the plant as fuel. Generally considered, these requirements are accomplished by further treating that portion ofthe gas stream going. to purge from line 36, so as to adjust the concentration of inert gases relative to combustible gases. v Q 1 One portion of the gasstream in line 36 is returned 3 hrough line 37, to line 2 2 to be recycled by compressor 53 through line 24 to the feed stream. A second portion )f the separating zone eflluent is discharged through line 8, and subjected to cooling by suitable means, typically )y passage through heat exchangers 39 and 40 to be iashed past valve 41 into chamber 42. The purged gas )verhead therefrom is discharged through line 43 conaining back pressure valve 44, through exchanger 39 and n indirect heat exchange With the feed stream in the cfrigeration zone 12, to be discharged at 45 to fuel or rther suitable disposition. The condensate in chamber 42. s withdrawn through line 48 past liquid level controlled al ve 47 for heat interchange with the gas streams in exha'nger 59, thence to be discharged through line 49 into he chamber 19 overhead in line 20;

In so processing the gas stream in line 38, we mainain by back pressure control at valve 44, a pressure aetween that existing in chambers 16 and 19 such that he ratio of inerts to combustibles in the overhead from :hamber 42 will be that required to remove from the ystem the inerts entering with the feed gas stream, while imiting the combustibles, esssentially methane, in the )urged gas to an economical quantity. The purged gas is :ooled in exchanger 39 where advantage is taken of the efrigeration potential in the liquid and gaseous effluent rom chamber 42. This supplies most of the refrigeration equired to adequately condense thepurged gas stream. Fhe balance of the necessary refrigeration may besupxlied by flashing at valve 471, a stream of zone 16 conlensate taken therefrom through line 481 The flashed tream is discharged from exchanger 40 through line 491' nto line 48. If the temperature level so reached in exhanger 40 is insufficient to produce the desired separation If nitrogen, part of the liquid fromchamber 19 may be ubstituted for liquid taken from chamber 16.

The cold, liquefied purge gas isfiashed at valve 41 to reduce a vapor in line 43 with the required percent of nerts. The liquid withdrawn'from chamber 42 undergoes xpansion at valve 47, thus lowering the temperature of he stream passing through exchanger 39' in precooling elation with the gas stream in line 38.

It may be mentioned generally that depending upon 'arious factors such as the ratio of inerts to fuel in the eed stream, temperature and pressure and conditions in be earlier stages, the described purged gas processing ystem can be located between chambers 19 and 27, or letween chamber '27 and the storage tank 32, as may be most economical.-

In the following tabulation we show typical stream lows (based e.g. on 100 mol's of feed at point A) and posiions at locations designated at A to F in the drawing, vhere the feed gas contains around 83 percent methane". Natural gases, or other light hydrocarbon gases, treatable a accordance with the invention'may contain, as illustraive around 50 to 98 percent methane with relatively small mounts of heavier hydrocarbons, and roughly 2 to 30 Iercent inerts.)

The system is flexible and simple to operate since there 4. are no distillation columns whose proper operation is sensitive to liquid or vapor overloadings, and there are no moving parts to maintain. The adaptability of the system to varying quantities of inerts in the feed gas is particularly noteworthy. The quantity of inerts in the purge gas can be varied simply by adjusting the back pressure at valve 44. For even greater flexibility, duplicate piping can be arranged between expansion zones 19 and 27 so that the purge stream may be extracted from line 20 if desired.

We claim:

1. The process that includes cooling a feed stream of compressed natural or other gas containing methane and nitrogen in a refrigeration zone below the boiling temperature of methane, passing and'expen'ding the cooled effluent from said zone into a separating zone, withdrawing liquid methane from said separating zone and further cooling the withdrawn methane, withdrawing fromthe last mentioned zone a separated nitrogen and methane gas stream, compressing and returning a portion of said separated gas stream to the feed stream in said refrigeration zone through passage means leading from the separating zone to the refrigeration z'one, passing the remainder of saidgas stream at reduced pressure into a cooling and expansion zone wherein the components of said gas stream exist as nitrogen-methane gas and essentially methane condensate, cooling said separated gas stream going to said cooling and, expansion zone by passage through, an exchange zone, separately withdrawing methane condensate and nitrogen-methane gas from said cooling and expansion zone, removing a stream of the withdrawn nitrogen-methane gas as purge gas from the system, and variably restricting the last mentioned stream to impose back pressure in said cooling and expansion zone varying in accordance with variations in said restriction of the last mentioned stream to thereby vary methane vaporization in the cooling and expansion zone and therefore the methane content in the purge gas stream. 4

2. The process according to claim 1, in which the pressure of said cooled separated gas stream is reduced after passage through said ex'changczone. V M

3. The process according to claim 1, in which said separated gas stream going to the cooling and expansion zone is indirectly heat-interchanged with a stream of said essentially methane condensate withdrawn from said cooling'and expansion zone. a V p 4. The process according to clairnl, in which said separated gas stream going to the cooling and expansion zone is heat-interchanged in said exchange zone with said purge gas from the cooling and expansion zone.

5 The process according to claim 4, in which the pres sure of the purge gas is controllably reduced b etween the cooling and expansion zone and said exchange zone.

6. The process according to claim 5, in which the purge gas is heat interchanged with thefeed gas in said refrigeration zone. 7 g I 7. The process that includes cooling compressed natural gas containing methane and nitrogen in a refrigeration zone below the boiling temperature of methane, passing and expanding the cooled efiiuent from said zone into a separating zone, Withdrawing liquid methane from said separating zone and further cooling the withdrawn,

methane by flashing the methane at reducing pressures within a series of flash zones with formation of methane vapor as a result of the reducing pressures, compressing and recycling the resultin'g methane vapor to the feed gas stream and said refrigeration zone, withdrawing from the separating zone a separated nitrogen and methane gas stream, compressing and returning a portion of said separated gas stream to said refrigeration zone, passing the remainder of said gas stream at reduced pressure into a cooling and expansion zone wherein the components of said gas stream exist as a nitrogen- 'methane gas component and an essentially methane condensate component, separately removing from said cooling and expansion zone said components as purge gas and condensate streams, thermally interchanging one of said components with said separated gas stream going to said cooling and expansion zone, combining the interchanged methane condensate component with methane withdrawn from said separating zone, and variably restricting said purge gas stream to impose back pressure in said cooling and expansion zone varying in accordance with variations in said restriction of the purge gas stream to thereby vary methane vaporization in the cooling and expansion zone and therefore the methane content in the purge gas stream.

8. The process according to claim 7, in which both of said components are heat-interchanged with said separated gas stream going to the cooling and expansion zone.

9. The process according to claim 7, in which the said combining of interchanged methane from the cooling and expansion zone is by addition of such methane to methane vapors from one of said flash zones.

10. The process according to claim 7, in which said separated gas stream going to thecooling and expansion zone is heat interchanged with cooled liquid methane derived from said separating zone and the interchanged methane is combined with the vapor from one of said flash zones.

References Cited by the Examiner UNITED STATES PATENTS 2,162,963 6/39 McKittrick 62-23 2,557,171 6/51 Bodle 6223- 2,601,599 6/52 Denning 6223 2,771,149 11/56 Miller 6237 2,880,592 4/59 Davison 62-23 2,940,271 6/60 Jackson 62-23 2,959,020 11/ 60 Knapp 62-9 2,973,834 3/61 Cicalese 62-.23 3,062,015

11/62 Cost 62--23 NORMAN YUDKOFF, Primary Examiner. V

20 EDWARD J. MICHAEL, Examiner. 

1. THE PROCESS THAT INCLUDES COOLING A FEED STREAM OF COMPRESSED NATURAL OR OTHER GAS CONTAINING METHANE AND NITROGEN IN A REFRIGERATION ZONE BELOW THE BOILING TEMPERATURE OF METHANE, PASSING AND EXPENDING THE COOLED EFFLUENT FROM SAID ZONE INTO A SEPARATING ZONE, WITHDRAWING LIQUID METHANE FROM SAID SEPARATING ZONE AND FURTHER COOLING THE WITHDRAWN METHANE, WITHDRAWING FROM THE LAST MENTIONED ZONE A SEPARATED NITROGEN AND METHANE GAS STREAM, COMPRESSING AND RETURNING A PORTION OF SAID SEPARATED GAS STREAM TO THE FEED STREAM IN SAID REFRIGERATION ZONE THROUGH PASSAGE MEANS LEADING FROM THE SEPARATING ZONE TO THE REFRIGERATION ZONE, PASSING THE REMAINDER OF SAID GAS STREAM AT REDUCED PRESSURE INTO A COOLING AND EXPANSION ZONE WHEREIN THE COMPONENTS OF SAID GAS STREAM EXIT AS NITROGEN-METHANE GAS AND ESSENTIALLY METHANE CONDENSATE, COOLING SAID SEPARATED GAS STREAM GOING TO SAID COOLING AND EXPANSION ZONE BY PASSAGE THROUGH AN EXCHANGE ZONE, SEPARATELY WITHDRAWING METHANE CONDENSATE AND NITROGEN-METHANE GAS FROM SAID COOLING AND EXPANSION ZONE, REMOVING A STREAM OF THE WITHDRAWN NITROGEN-METHANE GAS AS PURGE GAS FROM THE SYSTEM, AND VARIABLY RESTRICTING THE LAST MENTIONED STREAM TO IMPOSE BACK PRESSURE IN SAID COOLING AND EXPANSION ZONE VARYING IN ACCORDANCE WITH VARIATIONS IN SAID RESTRICTION OF THE LAST MENTIONED STREAM TO THEREBY VARY METHANE VAPORIZATION IN THE COOLING AND EXPANSION ZONE AND THEREBY THE METHANE CONTENT IN THE PURGE GAS STREAM. 